Inherited diseases caused by mutations on the X chromosome are generally characterised by the affected status of carrier males and sparing of carrier females. EFMR (Epilepsy and Mental Retardation limited to Females) is a unique X-linked condition which, by contrast, spares carrier males and is expressed in females (Ryan S G et al., 1997). EFMR is a rare condition characterised by seizure onset in early childhood (6-36 months) and cognitive impairment. The phenotype is restricted to females with males apparently spared, demonstrating normal cognitive function and absence of seizures.
Prior to the studies described herein, the cause of EFMR was unknown, with the presence of EFMR not previously attributed to any specific genetic factor. The studies described herein now identify the protocadherin 19 (PCDH19) gene as responsible for EFMR.
By the systematic re-sequencing of 737 X-linked genes, seven different mutations in the PCDH19 gene were identified in seven unrelated families with EFMR. Five of these mutations result in the introduction of a premature termination codon resulting in non-functional PCDH19 mRNA that is degraded by nonsense mediated decay (NMD) processes. The two other mutations have been determined to be missense mutations and are likely to affect adhesiveness of the PCDH19 protein through impaired calcium binding.
PCDH19 is the first cadherin to be implicated in epilepsy and mental retardation. The expression analysis described herein shows a role for PCDH19 in normal neuronal development. A mechanism of phenotype rescue that saves transmitting males (ie carrier males) from clinically expressing the disorder is suspected, through a related male-specific human gene, protocadherin 11Y (PCDH11Y) (Blanco P et al., 2000). This mechanism is consistent with the remarkable mode of inheritance observed in EFMR.
The studies described herein have identified nucleotide and amino acid sequences corresponding to a complete PCDH19 open reading frame (ORF) as well as mutant sequences encoding non-functional PCDH19 mRNA or non-functional PCDH19 protein. These are shown to be related to illnesses associated with PCDH19 protein deficiency or altered function such as epilepsy and mental retardation, in particular EFMR. Further, male carriers of the PCDH19 deficient genotype have been shown to be rescued from the disease phenotype by the male-specific protocadherin PCDH11Y.
The identification of the complete PCDH19 ORF and the identification of mutations in the nucleotide sequence causing a disease state provide for methods for diagnosis of illnesses related to PCDH19 protein deficiency or altered PCDH19 protein function, methods for the identification of a predisposition to such illnesses, methods of screening to identify carriers of such illnesses methods, and agents for the therapeutic or prophylactic treatment of PCDH19 deficiency.